This invention relates to an X-ray tube apparatus and, more particularly, to an X-ray tube apparatus having a rotating anode X-ray tube.
Generally, an X-ray tube apparatus is employed for X-ray examinations of patients for medical purposes. The X-ray tube apparatus that is used for such medical purposes as the examination of the stomach has a rotating anode X-ray tube. The rotating anode X-ray tube has a vacuum envelope, in which a cathode assembly and an anode target are hermetically accommodated. The anode target has a target disk. The target surface of the target disk and the cathode assembly are disposed in such a manner that they are off-set from the axis of the vacuum envelope and that they oppose each other. The target disk is connected in a rotor in the vacuum envelope. In operation, the rotor is driven by electromagnetic induction that is produced by a stator disposed outside the vacuum envelope.
The cathode assembly of the rotating anode X-ray tube noted above has a focusing electrode having a focusing dimple. A tungsten coil filament which can emit electrons is provided at the bottom of the focusing dimple. Usually, the focusing electrode is kept to the same potential as that of the filament. Electrons emitted from the filament are focused on the target surface by a focusing field that is formed in the focusing dimple by a high potential applied to an anode target.
In this cathode assembly, however, the coil filament partly projects into the focusing dimple of the focusing electrode. This arrangement is adopted so that the tube current is kept within the limitted temperature range of the tube and the projecting portion thereof in the focusing dimple has an effect of intensifying the neighboring electric field. With the protruding portion of the filament an equipotential surface in the vicinity thereof protrudes toward the target surface at a central part of the filament. Electrons emitted substantially from the side walls of the filament are directed sidewise of the focusing dimple due to a electric field produced in a zone between a bottom portion of the focusing dimple and the protruding portion of the filament into the focusing dimple, while also they are directed toward the center of the dimple due to a field in the vicinity of the open end of the focusing dimple. Therefore, electrons emitted from the side walls of the filament and those emitted from the central portion of the filament cannot be focused on the same spot. More specifically, the loci of electrons emitted from opposite side walls of the filament cross one another considerably before they reach the target. Therefore, there results a double-humped electron density distribution over the target surface area where substantially all the emitted electrons are focused with respect to the axis of the electron beam.
With the cathode assembly of the above structure, therefore, the electrons emitted from the filament cannot be focused on a sufficiently small spot on the target by the focusing electrode. In order to obtain a sufficiently small focal spot on the target surface, it is necessary to use a filament having a small size. Such a small size filament, however, requires an increased operating temperature to obtain a sufficiently high density of emitted electrons. The prior art X-ray tube, therefore, has problems in respect of the limitation of the tube current.
Further, electrons impinged on the target is not equal to each other in the progressive directions thereof, so that it is impossible to obtain a sufficiently small focal spot and a desired electron density distribution, that is, it is impossible to obtain a sufficiently high resolution.
In order to overcome the above difficulties, it is designed to employ a flattened filament. The use of such a filament is disclosed in Japanese Laid-Open No. 68056/1980 (or No. 13658/1982).
In the disclosed X-ray tube, a filament in the form of a flat strip is used. This filament has a flat central portion serving as electron emission section and a pair of leg portions. These leg portions are provided at the ends of the flat central portion (or meandering portion), the leg portions being bent at right angles from the ends. The leg portions are supported with supports. It is heated directly by an electric current passed through it, so that it emits electrons from its central portion.
However, the filament is elevated to a high temperature by the current passed through it, so that its central portion, i.e., electron emission section, is greatly deformed, i.e., curved outwardly toward the target surface, due to its thermal expansion. This deformation of the filament results in a great deviation of the focal point of electrons emitted from the central portion of the filament on the target surface.
Further, the leg portions of the filament are also deformed outwardly. This means that changes in the filament temperature change the position of the electron emission section of the filament in the direction of the tube axis, thus changing the positional relation between the focusing dimple of the focusing electrode and electron emission section of the filament. This would greatly change not only the shape of the electron beam spot on the target surface but also the width of the electron beam and the amount thereof reaching the target surface. These variations are undesired from the stantpoint of accurate control of the resolution or amount of X rays.
Meanwhile, United States Pat. No. 4,126,805 discloses an X-ray tube provided with means for preventing the displacement of the electron emission surface. The disclosed displacement prevention means, however, has a very complicated structure.